This invention relates to the field of fire or explosion suppression in aircraft fuel tanks, and more particularly to a novel apparatus or device for that purpose which can be readily incorporated into new construction or can be easily retrofitted to existing aircraft without requiring mounting or structural modifications internally of a fuel cell, tank, or compartment and without requiring any new penetrations of a wall, prior to an event calling for flame or explosion suppression.
The single most important cause of aircraft losses in combat is fire or explosion in the fuel system. As an aircraft's fuel tanks empty, a highly explosive fuel-air mixture is left in the tanks which can be ignited by an incoming projectile or missile warhead fragments. The resulting explosion generates severe overpressures which cause catastrophic structural failure. A number of systems have been developed to interrupt this sequence either by reducing the ullage oxygen content to reduce its flammability, by incorporating foams or similar materials to retard the flame front, or by chemically suppressing an explosion with an inerting agent, typically "HALON". Each of these approaches has problems. Reducing oxygen content sufficiently to prevent vapor ignition can be done using systems with molecular sieves or semipermeable membranes to selectively pass nitrogen into the ullage spaces while trapping oxygen, but such systems tend to respond unfavorably to rapid altitude changes common to combat aircraft, with the result that they must be made excessively large and heavy to provide adequate inerting during maneuvering flight. The same results can be achieved by purging the ullage with nitrogen from a pressurized or liquid N.sub.2 reservoir before entering combat, but the provisioning and handling of cryogenic liquids in a combat environment coupled with the necessity for refilling after each flight make this type of storage impractical, while pressurized gaseous nitrogen is excessively bulky. The use of foam, while practical and well-proven, involves significant fuel loss (about 7%) and weight penalties. The class of "HALON" inertant systems appear to offer the most promise for future systems since they involve substantially less weight and fuel volume loss penalties than the foams, while providing more reliable protection than the nitrogen generators and easier servicing than the liquid nitrogem systems.
The use of inertant systems for aircraft fire protection has a fairly long history. Explosion suppression systems, however, are a more recent concept and have not been completely proven. Two approaches to the problem are possible. In the first, adopted by the F-16, a reservoir of a fire suppressant liquid under pressure, such as that sold under the name "HALON", is activated by the pilot as he crosses into a combat area and is released into the ullage volume to provide a limited-duration inerting. This requires that the inertant bottle be refilled after each mission, significantly increasing turnaround time. Additionally, the aircraft remains vulnerable to surprise hits taken before the pilot has activated the system, or to second hits that may occur after the inerting gas has spilled overboard through a first-hit entry or exit hole. To overcome these deficiencies a second type of system as been previously proposed in which the inerting agent is stored in numerous small bottles mounted inside the tank. When needed, the agent is released only into the area in which vapor ignition has occurred. Such a system would employ optical sensors to detect the light flash of vapor ignition, a control unit to process the sensor signal, and "HALON" bottles in each ullage compartment to be triggered as needed by the control unit. A distribution tube network would be used to transfer each bottle's output through structural partitions to ensure that sufficient inertant reaches all parts of the ullage volume to suppress an explosion before destructive overpressures occur. Advantages include relatively low weight and small fuel loss penalties, low maintenance, low logistics penalties, automatic operation when needed, and multiple hit protection. However, a relatively large number of penetrations must be made into the pressurized fuel tank for interconnections between the components inside and outside of the tank. Structural modifications must be made to tank interior components to allow cutouts for distribution tubing. In addition, the "HALON" bottles fitted inside the tank contain both electrical wiring which must be prevented from sparking and explosive squibs for bottle actuation which could be inadvertently fired by a spurious signal. Accidential bottle firing could cause structural damage if the tank was comletely full. It would also require opening the fuel tank for replacement of the fired bottle, which could be a time-consuming task given the limited access available in most fuel tanks and the likelihood of creating leaks when the tank is closed. Finally, recent tests of such a system designed for a wing tank have shown that the detection of an explosion and actuation of a "HALON" bottle can be accomplished in a satisfactory time, but the use of distribution tubing to direct the intertant to all hidden areas of the compartment introduces unacceptable time delays.
With the foregoing in mind, it is a principal object of the invention to provide an improved fire or explosion suppression device or system for aircraft fuel tanks.
Another object of the invention is to provide fire or explosion apparatus that overcomes the major operational deficiencies and shortcomings of the prior art systems discussed above, and yet is reliable in operation, economical to implement, and which can be easily retrofitted to most fuel tank configurations with little or no tank intrusion during such retrofitting, and concomittently can be accessed for servicing without entry into the fuel tanks.
As another object, the invention aims to accomplish the foregoing through the use of a novel suppressant agent containing bottle or flask having a dispensing duct and a shaped or directed charge operable to remove a duct seal and penetrate the tank wall so as to release the suppressing agent into the tank at an optimal location, and directed so as to suppress the initiating fire or explosion.
Yet another object of the invention is to provide a fire suppressant device that can be incorporated with a number of like devices into a fire or explosion suppressant system of the type wherein electro-optical sensing means are used to detect an incipient explosion and cooperate with a control logic to detonate the appropriate explosive charge or charges of one or more of the explosion extinguishing bottles suitably distributed on the walls of the aircraft or other fuel tanks.
Other objects and many of the attendant advantages will be readily appreciated as the subject invention becomes better understood by reference to the following detailed description, when considered in conjunction with the accompanying drawings.